Image forming apparatus containing a recording sheet guide member for bending the recording sheet at the center

ABSTRACT

According to an example structure, an image forming apparatus includes a belt having a conveying surface on which a recording sheet is conveyed; an image forming unit disposed opposite to the belt and forming an image on a surface of the recording sheet conveyed by the belt; a conveying unit conveying the recording sheet onto the belt; and a guiding member disposed opposite to the conveying surface of the belt and configured to at least partly guide the recording sheet toward the conveying surface of the belt while the guiding member comes into contact with a first back surface opposite to a surface of which the recording sheet comes into contact with the conveying surface of the belt. The recording sheet is guided by the guiding member while being bent at a center of tip end portion with respect to a width direction orthogonal to a conveying direction of the recording sheet so as to protrude toward the conveying surface of the belt.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims the benefit of priority from theprior Japanese Patent Applications No. 2006-089097, filed on Mar. 28,2006 and No. 2006-323917, filed on Nov. 30, 2006; the entire contents ofwhich are incorporated herein by reference.

TECHNICAL FIELD

An aspect of the present invention relates to an image formingapparatus, more specifically relates to a technique for guiding arecording sheet onto a belt that conveys a recording sheet in the imageforming apparatus.

BACKGROUND

As an image forming apparatus, there is known one including a belt forconveying a recording sheet and an image forming unit which is disposedso as to face a conveying surface of the belt and forms an image on thesurface of a recording sheet conveyed by the belt.

In such an image forming apparatus, when a recording sheet fed from theupstream side of the recording sheet conveying direction of the beltcomes into contact with the belt, air may be trapped between therecording sheet and the conveying surface of the belt and the recordingsheet may partially float from the conveying surface of the belt. If therecording sheet is conveyed in this state to the image forming unit andan image is formed on the surface thereof, the quality of the formedimage may lower.

For example, in an electrophotographic image forming apparatus, theimage forming unit is configured by a photosensitive member, and arecording sheet is conveyed between the photosensitive member and theconveying surface of the belt, and a transfer bias is applied to atransfer unit and a developing image is transferred (formed) onto thesurface of the recording sheet. However, when the transfer is performedin a state that the recording sheet floats from the conveying surface ofthe belt, there is a possibility that the transfer bias is notsufficiently applied to the floating portion and a so-called void wherea developing image is not transferred occurs there, and the imagequality lowers.

Therefore, a guiding member is provided which is disposed at an upstreamside of the belt in the recording sheet conveying direction; and theguiding member comes into contact with a surface of a recording sheetfacing the conveying surface of the belt and guides the tip end portionof the recording sheet to the belt while curving a central portion ofthe recording sheet in the width direction orthogonal to the conveyingdirection of the recording sheet so as to protrude toward the conveyingsurface of the belt.

By the above-described configuration, when a recording sheet is guidedby the guiding member while being curved, the recording sheet can beguided to the belt so that, first, the widthwise central portion of therecording sheet curved and protruded toward the conveying surface sidecomes into contact with the conveying surface of the belt, and whilebeing conveyed, the contact area gradually spreads toward both end sidesin the width direction of the recording sheet and air between theconveying surface of the belt and the recording sheet is expelled toboth ends. Thus, by expelling air trapped between the recording sheetand the conveying surface of the belt to both end sides, air can beprevented from being trapped between the recording sheet and theconveying surface of the belt and partially floating the recording sheetfrom the conveying surface of the belt and lowering the image quality.

SUMMARY

However, according to the above-described related art, the guidingmember is in contact with a surface of a recording sheet facing theconveying surface of the belt, so that a tip end portion of therecording sheet may warp in a direction of separating from the conveyingsurface of the belt due to a drag from the guiding member. In addition,when the recording sheet comes into contact with the guiding member in astate that the tip end portion of the recording sheet is curled so as toseparate from the conveying surface of the belt, the recording sheetwarps more from the conveying surface of the belt. When the recordingsheet comes into contact with the belt in a state that the tip endthereof thus warps from the conveying surface of the belt, the tip endportion of the recording sheet does not come into close contact with theconveying surface of the belt and floats therefrom, and air may betrapped between the recording sheet and the conveying surface and lowersthe image quality.

The present invention has been made in view of the above circumstancesand provides an image forming apparatus. According to an aspect of theinvention, there is provided an image forming apparatus capable ofpreventing a lowering in image quality.

According to another aspect of the invention, an image forming apparatusincluding: a belt having a conveying surface on which a recording sheetis conveyed; an image forming unit disposed opposite to the belt, theimage forming unit forming an image on a surface of the recording sheetconveyed by the belt; a conveying unit conveying the recording sheetonto the belt; and a guiding member disposed opposite to the conveyingsurface of the belt and configured to at least partly guide therecording sheet toward the conveying surface of the belt while theguiding member comes into contact with a first back surface opposite toa surface of which the recording sheet comes into contact with theconveying surface of the belt, the recording sheet being guided by theguiding member while the recording sheet is bent at a center of tip endportion thereof with respect to a width direction orthogonal to aconveying direction of the recording sheet so as to protrude toward theconveying surface of the belt.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings;

FIG. 1 is an exemplary sectional side view showing a generalconstruction of a laser printer according to an example structure of theimage forming apparatus;

FIG. 2 is an exemplary sectional side view showing a state that a paperfeeding tray is drawn out of the main body casing;

FIG. 3 is an exemplary sectional side view showing a state that theconveying unit is drawn out of the main body casing;

FIG. 4 is an exemplary sectional side view showing a state that a coveris opened;

FIG. 5 is an exemplary sectional side view showing a state that the beltunit is detached from the conveying unit;

FIG. 6 is an exemplary enlarged sectional side view of the vicinity ofthe guide member;

FIG. 7 is an enlarged sectional side view of the vicinity of the guidemember;

FIG. 8 is an exemplary sectional view along X-X of FIG. 6;

FIG. 9 is an exemplary schematic view showing the contact of the tip endportion of the sheet with the conveying surface of the belt;

FIG. 10 is an exemplary enlarged sectional side view of the vicinity ofthe guide member;

FIG. 11 is an exemplary sectional view along X-X of FIG. 10;

FIG. 12 is an exemplary enlarged sectional side view of the vicinity ofthe guide member;

FIG. 13 is an exemplary enlarged sectional side view of the vicinity ofthe guide member;

FIG. 14 is an exemplary sectional side view showing a generalconstruction of a laser printer according to another example structureof the image forming apparatus;

FIG. 15 is an exemplary sectional side view showing a generalconstruction of a laser printer according to another example structureof the image forming apparatus;

FIG. 16 is an exemplary enlarged sectional side view of the vicinity ofthe guide member;

FIG. 17 is an exemplary sectional side view of a general construction ofa laser printer according to an example structure of the image formingapparatus;

FIG. 18 is an exemplary sectional side view showing a state that theguiding member 700 is at the position projecting toward the surface ofthe sheet 82;

FIG. 19 is an exemplary sectional side view showing a state that theguiding member 700 is at the position retracted from the surface of thesheet 82 more than a projecting position of the guiding member;

FIG. 20 is an exemplary perspective view of the guiding member 700 fromthe lower side;

FIG. 21 is an exemplary view of the guiding member 700 on the arrow A ofFIG. 18;

FIG. 22 is an exemplary view of the support member 800 on the arrow A ofFIG. 20;

FIG. 23 is an exemplary sectional side view of a laser printer 80;

FIG. 24A is an exemplary sectional side view showing a state that theprotrusion rib 900 is at the “projecting position,” and FIG. 24B is anexemplary sectional side view showing a state that the protrusion rib 90is at the “retracted position;”

FIG. 25A is an exemplary sectional view along the arrow P-P of FIG. 24A,showing the state that the protrusion rib 900 is at the “projectingposition,” and FIG. 25B is an exemplary sectional view along the arrowP-P of FIG. 24B, showing the state that the protrusion rib 900 is at the“retracted position;”

FIG. 26 is an exemplary block diagram of a laser printer 80;

FIG. 27 is an exemplary flowchart showing solenoid control processing;

FIG. 28 is an exemplary sectional side view of a laser printer 80; and

FIG. 29 is an exemplary flow chart showing solenoid control processing.

DESCRIPTION OF THE EXAMPLE STRUCTURES

Hereinafter, “elasticity” in the recording sheet means a level of theelastic force of the recording sheet, or difficulty in deformation. Therecording sheet being high in elasticity is hardly creased.

Next, an example structure of the invention will be described withreference to FIG. 1 through FIG. 9.

A laser printer 1 is a direct tandem color laser printer includingphotosensitive drums 42 as an example of four photosensitive memberscorresponding to the colors of black, cyan, magenta, and yellow. Thelaser printer 1 includes, in the main body casing 2, a conveying unit 4conveying a sheet 3 as an example of the recording sheet, an imageforming unit 5 as an example of the image forming unit for forming animage on the sheet 3 conveyed by the conveying unit 4, and so on. In thefollowing description, the right side with respect to the paper surfaceof FIG. 1 is defined as “front side,” the left side with respect to thepaper surface is defined as “rear side,” the upper side with respect tothe paper surface is defined as “upper side,” and the lower side withrespect to the paper surface is defined as “lower side.”

The conveying unit 4 is provided in the lower side of the main bodycasing 2. The conveying unit 4 can be drawn forward from the main bodycasing 2. The conveying unit 4 has a unit frame 7. Under the unit frame7, a shallow-tray-shaped paper feeding tray 8 in which sheets 3 to befed to the image forming unit 5 can be stacked is detachably attached tothe unit frame 7. A front wall 8A is provided on the front end of thepaper feeding tray 8. The front wall 8A is disposed at the lowestposition of the front surface of the main body casing 2. By drawing thefront wall 8A forward, as shown in FIG. 2, the paper feeding tray 8 canbe detached from the unit frame 7 and independently drawn forward of themain body casing 2.

A paper presser (not shown) is provided on the bottom surface of thepaper feeding tray 8. The paper presser can tilt so as to lift the frontend side. A pickup roller 9 held by the unit frame 7 is provided at afront end upper position of the paper feeding tray 8. A paper feedroller 10 held by the unit frame 7 is provided in front of the front endupper position of the paper feeding tray 8. A separating pad 12 isprovided on the front side of the paper feeding tray 8. The separatingpad 12 is brought into contact with the paper feed roller 10 by anurging force of a spring 11. A pair of paper dust removing rollers 13Aand 13B are provided on a diagonally upper front side of the paper feedroller 10. One paper dust removing roller 13A is attached to the unitframe 7, and the other paper dust removing roller 13B is attached to anupper end of the back surface of the front wall 8A of the paper feedingtray 8.

A sheet 3 at the top in the paper feeding tray 8 is pressed against thepickup roller 9 by an urging force of the paper presser, and the sheet 3is conveyed between the paper feed roller 10 and the separating pad 12by rotation of the pickup roller 9. The sheet 3 is separated alone whensandwiched between the paper feed roller 10 and the separating pad 12and fed to the diagonally upper front side. Paper dust is removed fromthe sheet 3 by the pair of paper dust removing rollers 13A and 13B, andthen the sheet 3 is conveyed to resist rollers 17A and 17B serving asconveying rollers through a tray side paper feed path 19 (see FIG. 6)formed substantially upward from the paper dust removing rollers 13A and1B. The conveying unit is configured by, for example, the resist rollers17A and 17B and a guide member described later.

On the front end portion of the conveying unit 4, a front wall 15 isprovided so as to continuously flush with the front face of the mainbody casing 2 and the front wall 8A of the paper feeding tray 8. Theconveying unit 4 can be entirely drawn forward from the main body casing2 as shown in FIG. 3 by drawing the front wall 15 forward. In a lowerportion of the front wall 15, a manual paper feed port 18A (see FIG. 6)through which the sheet 3 to be manually fed can be inserted is opened.On the inner side of the front wall 15, a pair of resist rollers 17A and17B serving as the conveying rollers are provided. A manual feed sidepaper feed path 18 (see FIG. 6) continued from the manual paper feedport 18A and a tray side paper feed path 19 extending substantiallyupward from the paper dust removing rollers 13A and 13B join at aposition immediately before the resist rollers 17A and 17B. On theresist rollers 17A and 17B, a sheet 3 fed from both paper feed paths 18and 19 is subjected to skew correction, and then fed to the uppersurface of the belt 29 through the paper feed path 20. The paper feedpath 20 is constructed between an inner guide member 21 formedintegrally on the upper surface of the unit frame 7 and a guide member22 attached so as to face said guide member 2 at a higher position, andthe paper feed path is curved so as to be convex upward as a whole. Theguide member 22 and other elements around the guide member 22 will bedescribed in detail later.

The unit frame 7 has a tray-shaped belt unit attaching part 24 openedupward at a rear position of the inner guide member 21, and a belt unit25 detachably attached thereto. The belt unit 25 has a box-shaped beltframe 26 opened upward (see FIG. 6, not shown in FIG. 1 through FIG. 5),and the respective parts of the belt unit 25 are housed inside the beltframe. The belt unit 25 has a pair of belt support rollers 27 and 28disposed in parallel while spaced forward and rearward from each otherand a belt 29 laid across the belt support rollers 27 and 28. The beltmoves by a rotation of the rear-side belt support roller 28 driven by amotor (not shown). The front-side belt support roller 27 is in aposition slightly higher than the rear-side belt support roller 28. Theconveying surface 29A (upper surface) of the belt 29 on which the sheet3 is conveyed is inclined slightly downward toward the rear end morethan the horizontal direction. On the inner side of the belt 29, fourtransfer units disposed so as to face the respective photosensitivedrums 42 of process cartridges 35 (described later) and transfer rollers31 serving as an electrostatic adsorbing unit are provided in line atpredetermined intervals in the front and rear direction. The belt 29 issandwiched between the photosensitive drums 42 and the correspondingtransfer rollers 31. A cleaning roller 32 cleaning remaining toneradhering to the surface of the belt 29 is provided under the belt 29.The sheet 3 fed out from the resist rollers 17A and 17B passes throughthe paper feed path 20 and comes into contact with the vicinity of thefront end of the conveying surface 29A of the belt 29. The sheet iselectrostatically adsorbed by the conveying surface 29A of the belt 29due to a transfer bias applied to the transfer roller 31 and conveyedrearward according to rotation of the belt 29.

An image forming unit 5 is provided above the belt unit 25 inside themain body casing 2. Four Scanners 34 serving as the exposing unit andfour process cartridges 35 forming images in magenta, yellow, cyan, andblack are arranged alternately in the front and rear direction. Thescanner 34 houses inside a scanner case 39 a polygon mirror 36reflecting a laser beam L emitted from a laser diode (not shown) so asto successively change its direction along a predetermined surface, aturning mirror 37 turning the laser beam L reflected by the polygonmirror 36 toward the photosensitive drum 42 of the process cartridge 35,and an fθ lens 38 provided in the light path of the laser beam L. Eachscanner case 39 has a substantially plate-like external form, and eachscanner case 39 is attached in an inclined posture so that its upper endside turns forward.

The process cartridge 35 has a photosensitive drum 42 and a scorotroncharger 43. The photosensitive drum 42 is rotatably provided with theprocess cartridge 35 on the lower side of the frame-shaped cartridgeframe 41. The scorotron charger 43 for evenly charging the surface ofthe photosensitive drum 42 is provided around the photosensitive drum42. A developing cartridge 44 serving as the developing unit isdetachably attached to the cartridge frame 41. The developing cartridge44 has a substantially box-shaped case 45 opened downward. The case 45is attached in an inclined posture so that its upper end side turnsforward. A toner containing chamber 47 containing toner in each color asa developer is formed on the upper side of the inside of the case 45.Inside the toner containing chamber 47, an agitator (not shown) foragitating the toner is rotatably provided. A supply roller 48, adeveloping roller 49 and a layer thickness restricting blade (not shown)are provided on the lower side of the toner containing chamber 47 insidethe case 45. The process cartridge 35 is detachable from the main bodycasing 2. The process cartridge 35 is detached to the diagonally upperfront side from the main body casing 2 and attached in the oppositedirection.

Toner released from the toner containing chamber 47 is supplied to thedeveloping roller 49 by rotation of the supply roller 48, and at thistime, frictionally positively charged between the supply roller 48 andthe developing roller 49. The toner supplied onto the developing roller49 enters between the tip end of the layer thickness restricting bladeand the developing roller 49 in accordance with rotation of thedeveloping roller 49, and carried on the developing roller 49 as a thinfilm with a fixed thickness. The surface of the photosensitive drum 42is evenly positively charged by the scorotron charger 43 in accordancewith its rotation, and then exposed by high-speed scanning of a laserbeam L from the scanner 34, and an electrostatic latent imagecorresponding to an image to be formed on the sheet 3 is formed.

Next, by rotation of the developing roller 49, when the toner carried onthe developing roller 49 and positively charged faces and comes intocontact with the photosensitive drum 42, the toner is supplied to theelectrostatic latent image formed on the surface of the photosensitivedrum 42. Accordingly, the electrostatic latent image on thephotosensitive drum 42 is visualized, and a toner image formed byreversal development is carried on the surface of the photosensitivedrum 42.

Thereafter, toner images carried on the surfaces of the respectivephotosensitive drums 42 are successively transferred onto the sheet 3due to a transfer bias applied to the transfer rollers 31 during passingof the sheet 3 conveyed by the belt 29 through transfer positionsbetween the photosensitive drums 42 and the transfer rollers 31. Thesheet 3 on which toner images are transferred is conveyed to a fixingdevice 51.

The fixing device 51 is disposed on the rear side of the belt 29 in themain body casing. The fixing device 51 includes a heating roller 52 anda pressurizing roller 53, etc., disposed facing each other, and heat-fixthe toner images transferred on the sheet 3 to the sheet surface. Theheat-fixed sheet 3 is conveyed to an eject roller 54 provided on theupper side of the main body casing 2 while U-turned back. On the uppersurface of the main body casing 2, a cover 55 which can open and closearound a hinge 55A is provided, and the upper surface of the cover 55 isformed into a discharge tray 56 in which image-formed sheets 3 ejectedfrom the eject roller 54 are stacked. By opening this cover 55, as shownin FIG. 4, each process cartridge 35 becomes exchangeable.

As described above, in the laser printer 1, in order from the lowerside, in the main body casing 2, the paper feeding tray 8, the belt unit25, the image forming unit 5, and the discharge tray 56 on the uppersurface of the main body casing 2 are stacked vertically, and aso-called S-shaped conveying path is formed in which a sheet fed outforward from the paper feeding tray 8 is conveyed rearward on the belt29 by being U-turned and further turned back again and reaches thedischarge tray 8 on the upper surface.

Next, the guide member 22 and the periphery thereof in the conveyingunit 4 will be described in greater detail with reference to FIG. 6 andFIG. 7.

The pair of resist rollers 17A and 17B are equal in outer diameter toeach other, and the nipping position 17C of the resist rollers 17A and17B is positioned near the extension of the conveying surface 29A of thebelt 29 (see FIG. 7). The front-side resist roller 17B is positionedslightly higher than the rear-side resist roller 17A, and an angle Abetween a line connecting the central axes of the resist rollers 17A and17B and the horizontal axis is set in a range of 0 degrees<A<90 degrees.That is, a feed-out direction D1 of the sheet 3 from the resist rollers17A and 17B (direction orthogonal to the line connecting the centralaxes) is inclined diagonally upward to the belt 29 side with respect tothe vertical axis. The above-described manual feed side paper feed path18 is horizontal near the manual paper feed port 18A, that is, theinserting direction D2 of the sheet 3 into the manual feed side paperfeed path 18 extends rearward almost horizontally. The manual feed sidepaper feed path 18 is curved to the diagonally upper rear side so thatits angle becomes closer to the feed-out direction D1 of the sheet 3from the resist rollers 17A a 17B toward the downstream side from themanual paper feed port 18A. A feed-out direction D3 of the sheet 3 fromthe pair of paper dust removing rollers 13A and 13B to the tray sidepaper feed path 19 (direction orthogonal to a line connecting thecentral axes of the paper dust removing rollers 13A and 13B) is slightlyinclined to the diagonally-upper front side more than the verticaldirection. The tray side paper feed path 19 is slightly curved to thediagonally upper rear side so that it has an angle that becomes closerto the feed-out direction D1 of the sheet 3 from the resist rollers 17Aand 17B toward the downstream side. The feed-out direction D1 of thesheet 3 from the resist rollers 17A and 17B is a direction in the middleof the sheet inserting direction D2 into the manual feed side paper feedpath 18 and the feed-out direction D3 of the sheet 3 from the paper dustremoving rollers 13A and 13B, so that curves of sheets 3 passing throughthe paper feed paths 18 and 19 are both reduced. Therefore, with thisconstruction, even when the apparatus is downsized, curvature of a sheet3 in each of the paper feed path 18 or 19 is reduced, and therefore,loads on the sheet 3 or the resist rollers 17A and 17B, etc., can besuppressed.

The feed-out direction D1 of the sheet 3 from the resist rollers 17A and17B is inclined toward the conveying direction of the sheet 3 on theconveying surface 29A more than a direction orthogonal to the conveyingsurface 29A of the belt 29. Herein, In case that the feed-out directionD1 of the sheet 3 from the resist rollers 17A and 17B is set to beorthogonal to the conveying surface 29A, the curvature of the sheet 3 inthe paper feed path 20 increases. When the feed-out direction D1 of thesheet 3 from the resist rollers 17A and 17B is set to be parallel to theconveying surface 29A, if the sheet 3 tries to curve by a predeterminedamount, the curvature at the curved portion of the sheet 3 alsoincreases. In comparison with the cases, according to the examplestructure of the invention, the curve (curvature) of the sheet 3 in thepaper feed path 20 can be reduced. Therefore, with this construction,even when the apparatus is reduced size, the curvature of the sheet 3 inthe paper feed path 20 can be reduced, and therefore, loads on the sheet3, the resist rollers 17A and 17B, and the belt 29, etc., can besuppressed.

The guide member 22 is made of a synthetic resin material, and the guidemember 22 has a plate-shaped part 58 facing the paper feed path 20. Thewidth of this plate-shaped part 58 is set to be equivalent to that ofthe belt frame 26 of the belt unit 25. On the front end lower part ofthe guide member 22, a pair of shafts 59 are projected on both left andright sides (both sides in the width direction), and each shaft 59 isheld by a bearing (not shown) provided on the unit frame 7 side. Theguide member 22 is attached to be rotatable around the shafts 59. On thelower surface of the plate-shaped part 58, a guide surface 60 which thetip end portion of a sheet 3 fed out from the resist rollers 17A and 17Bcan come into rubbing contact with is formed. This guide surface 60 isformed into a concave shape so that from the upstream side to thedownstream side in the sheet conveying direction, it gradually separatesfrom the conveying surface 29A, and then gradually approaches theconveying surface 29A in the direction orthogonal to the conveyingsurface 29A. On the downstream side end of the plate-shaped part 58,positioning protrusions 61 are projectedly provided downward from bothleft and right ends, and by bringing the tip end portions of thepositioning protrusions 61 into contact with the upper end face of theside wall 26A of the belt frame 26, the downstream side end of the guidemember 22 is positioned. On the downstream side end of the plate-shapedpart 58, a reinforcement edge 62 extending almost orthogonally to theplate-shaped part 58 is formed across the entire width on the oppositesurface side of the guide surface 60. Thereby, the strength of the belt29 side of the guide member 22 is secured.

Above the belt 29, a lower end 45A of the case 45 of the above-describedprocess cartridge 35 is disposed so as to face the conveying surface 29Aof the belt 29, and the downstream side end of the guide member 22enters between the lower end 45A of the case 45 and the conveyingsurface 29A. The downstream side end of the guide member 22 isrestricted by the lower end 45A of the case 45 from being displacedupward.

On the inner side of the belt 29, a transfer roller 31 for subsequenttransfer of a first color is disposed on the downstream side of thefront-side belt support roller 27 (belt support roller closest to theguide member). The belt side end of the guide member 22 is disposed at aposition more downstream than the supporting position of the beltsupport roller 27 on the conveying surface 29A of the belt 29 whileslightly spaced from the conveying surface 29A. The conveying speed Vr(almost equal to the peripheral velocity of the resist rollers 17A and17B) of the sheet 3 conveyed by the resist rollers 17A and 17B has arelationship of Vr>Vb with the conveying speed Vb (almost equal to themoving speed of the belt 29) of the sheet 3 by the belt 29.

The inner guide member 21 is formed integrally with the front side uppersurface of the unit frame 7, and the inner guide member 21 has a convexguide surface 63 which the sheet 3 can come into rubbing contact withand is curved into a convex shape so as to face the guide surface 60.Protrusion ribs 200 serving as the guiding member is integrally fixedwith the guide member 22, and the protrusion ribs 200 are disposedadjacent to the downstream side end of the guide surface 60 and disposedopposite to the conveying surface 29A of the belt 29.

Two protrusion ribs 200 are projected so as to come into contact with aconveyed sheet 3 at positions facing a width direction central portionof the guide member 22, that is, a width direction central portion ofthe conveyed sheet 3.

On both sides in the width direction of the protrusion ribs 200, aplurality of guide ribs 210 protruded by a protrusion amount smallerthan that of the protrusion ribs 200 are fixed to the guide member 22.

The tip end portions 201 of the protrusion ribs 200 are inclined so asto gradually increase their protrusion amounts from the upstream side tothe downstream side as shown in FIG. 6 and FIG. 7.

As shown in FIG. 6 through FIG. 8, a pair of the protrusion ribs 220 anda plurality of ribs 223 serve as an auxiliary guiding member. Both ribs220, 223 are formed on the inner guide member 21 and protruded whilefacing the guide member 22.

The pair of the protrusion ribs 220 is sandwiched between the pair ofprotrusion ribs 200 along the width direction as shown in FIG. 8. Theplurality of ribs 223 are fixed on both sides of the pair of protrusionribs 200 in the width direction.

When the sheet 3 is guided by the guide surface 60 toward the protrusionribs 200, the widthwise central portion of the surface of the sheet 3opposite the surface facing the conveying surface 29A of the belt 29comes into contact with the tip end portions 201 of the protrusion ribs200 and the widthwise central portion of the tip end portion of thesheet 3 curves so as to protrude toward the conveying surface 29A of thebelt 29. At this time, the ribs 220 and 223 come into contact with thesurface facing the conveying surface 29A of the sheet 3 to stabilize theconveyance of the sheet 3.

The tip end portion of the sheet 3 guided by the protrusion ribs 200moves toward the conveying surface 29A of the belt 29 while thewidthwise central portion protrudes toward the conveying surface 29A ofthe belt 29 as show in FIG. 9.

When the sheet 3 is guided while curved by the protrusion ribs 200, thesheet 3 can be guided to the conveying surface 29A of the belt 29.First, the curved widthwise central portion of the sheet 3 protrudestoward the conveying surface 29A of the belt 29 and comes into contactwith the conveying surface 29A of the belt 29. The contact area of thesheet 3 spreads to both ends of the width direction of the sheet 3 whilebeing conveyed, and air between the conveying surface 29A of the beltand the sheet 3 is released to both end sides.

The protrusion ribs 200 that guide the tip end portion of the sheet 3toward the conveying surface 29A of the belt 29 are in contact with thesurface of the sheet 3 opposite the surface facing the conveying surface29A of the belt 29, so that the tip end portion of the sheet 3 isrestricted from being warped in a direction of separating from theconveying surface 29A, and comes into contact with the conveying surface29A of the belt 29 more reliably than in the related-art. As a result,occurrence of void due to air trapped between the tip end portion of thesheet 3 and the conveying surface 29A of the belt 29 is prevented, andlowering in quality of an image formed on the sheet 3 can be prevented.

To curve the widthwise central portion of the tip end portion of thesheet 3, protrusion ribs 20 whose front end areas are small are used, sothat in comparison with a case where ribs whose contact portions withthe sheet spread in a sheet-like shape, the contact areas with the sheet3 become smaller, and the conveyance resistance to the sheet 3 can bereduced. As a result, the sheet 3 can be prevented from jamming near theprotrusion ribs 200.

The protruded front end portions 201 of the protrusion ribs 20 areinclined and protrusion amount of the protrusion ribs 20 graduallyincreases toward the downstream side from the upstream side.Accordingly, when the sheet 3 comes into contact with the protrusionribs 200, the conveyance resistance to the sheet 3 can be prevented fromsuddenly increasing. Near the protrusion ribs 200, the sheet 3 can beprevented from jamming.

By electrostatically adsorbing the sheet 3 by the conveying surface 29Aof the belt 29, the sheet 3 can be more reliably brought into closecontact with the conveying surface 29A of the belt 29. However, at thesame time, if air is trapped between the sheet 3 and the conveyingsurface 29A of the belt 29, the conveying surface 29A of the belt 29 andthe sheet 3 are in close contact with each other around the air, so thatthe air is hardly released. However, according to the example structureof the invention, as described above, when the sheet 3 comes intocontact with the conveying surface 29A of the belt 29, the air trappedbetween the sheet 3 and the conveying surface 29A of the belt 29 isreleased, so that air between the sheet 3 and the conveying surface 29Aof the belt 29 is not be trapped.

The tip end portion of the sheet 3 is fed to the guide surface 60recessed into a concave shape by the resist rollers 17A and 17B, so thatthe tip end portion of the sheet 3 becomes difficult to separate fromthe guide surface 60 during feeding to the downstream side of the guidesurface 60, so that the tip end portion of the sheet 3 can be morereliably guided to the protrusion ribs 200.

In addition, a speed of which the sheet 3 is conveyed by the resistrollers 17A and 17B is higher than a speed of which the sheet isconveyed on the belt 29, so that the sheet 3 slackens between the belt29 and the resist rollers 17A and 17B, and the surface of the sheet 3opposite the surface facing the conveying surface 29A of the belt 29more reliably comes into contact with the guide surface 60 recessed intoa concave shape. As a result, the surface is more reliably guided to theprotrusion ribs 200, whereby the widthwise central portion of thesurface is more reliably protruded toward the conveying surface 29A ofthe belt 29, and air trapped between the surface and the conveyingsurface 29A of the belt 29 can be more reliably prevented.

When the sheet 3 is fed out by the resist rollers 17A and 17B, the tipend portion of the sheet 3 comes into contact with the vicinity of theupstream side end of the guide surface 60. Then, as the resist rollers17A and 17B push the sheet 3, the sheet 3 moves to the downstream sidewhile its tip end portion rubs against the guide surface 60, andaccordingly, the tip end portion of the sheet 3 gradually changes itsdirection to the protrusion rib 200 side. Herein, in the examplestructure of the invention, the contact angle E of the tip end portionof the sheet 3 with the guide surface 60 is set to be equal to or lessthan 45 degrees. The contact angle E of the tip end portion of the sheet3 can be obtained from “E=90°−C” provided that an angle between the tipend portion of the sheet 3 and the normal of the guide surface 60 at thecontact point is defined as C. When the tip end portion of the sheet 3comes into contact with the guide surface 60 at a great angle, the loadon the sheet 3 increases and the tip end portion of the sheet 3 may bedamaged, however, according to this construction, the sheet 3 comes intocontact with the guide surface 60 always at a gentle angle, so that theload on the sheet 3 can be suppressed.

When the sheet 3 thus moves to the downstream side while its front endrubs against the guide surface 60, an intermediate portion in the sheet3 between its tip end portion and the nipping position 17C between theresist rollers 17A and 17B is curved so as to gradually become convexupward. Then, between the intermediate portion of the sheet 3 and theguide surface 60, a space S is formed due to the elasticity of the sheet3 (see the chain line in FIG. 6).

Then, when the tip end portion of the sheet 3 passes over the downstreamside end of the protrusion ribs 200 and comes into contact with theconveying surface 29A of the belt 29, the sheet 3 is electrostaticallyadsorbed by the conveying surface 29A of the belt 29 and conveyedrearward according to the movement of the belt 29. Herein, the sheet 3fed out from the protrusion ribs 200 comes into contact with theconveying surface 29A of the belt 29 from the diagonally upper side.That is, an angle F between the feed-out direction of the sheet 3 fromthe protrusion ribs 200 and the conveying surface 29A of the belt 29becomes 0<F<30 degrees. Therefore, the force for pushing-out the sheet 3from the resist rollers 17A and 17B act so as to press the sheet 3against the conveying surface 29A of the belt 29, and the sheet 3reliably comes into close contact with the conveying surface 29A of thebelt 29 without floating therefrom.

The tip end portion of the sheet 3 thus adsorbed by the surface of thebelt 29 is sandwiched between the photosensitive drum 42 on theimmediately downstream side and the transfer roller 31 for the firstcolor, and the first color is transferred here. Then, as the sheet 3advances to the downstream side according to the movement of the belt29, other colors are successively transferred by the respectivephotosensitive drums 42 and transfer rollers 31. Herein, the conveyingspeed Vr of the sheet 3 by the resist rollers 17A and 17B is higher thanthe conveying speed Vb of the sheet 3 on the belt 29, so that betweenthe resist rollers 17A and 17B and the belt 29, the sheet 3 graduallyslackens, and this slack is released to the space S formed between thesheet 3 and the guide surface 60. At this time, the sheet 3 is in aposture curved by the guide member 22 and the inner guide member 21, sothat in comparison with slackening from a planar posture, loads on thesheet 3 and the resist rollers 17A and 17B, etc., are smaller, so thatthe sheet can be easily slackened.

When the rear end of the sheet 3 passes through the nipping position 17Cof the resist rollers 17A and 17B, the rear end of the sheet 3 mayvibrate in the thickness direction. However, in the presentconstruction, such vibrations are suppressed by bringing the sheet 3into contact with the guide member 22 or the inner guide member 21disposed on both front and back surfaces. Therefore, it can be preventedthat the vibration on the rear end of the sheet 3 is transmitted to thetransfer position and causes color displacement.

FIG. 5 is a sectional side view showing a state that the belt unit isdetached from the conveying unit.

To perform jamming handling or replacement, etc., of the belt 29 in thislaser printer 1, first, as shown in FIG. 3, the conveying unit 4 isdrawn forward of the main body casing 2. Thereby, when the sheet 3 jamson the belt 29 or around the fixing device 51, this jamming can besolved. When the sheet 3 jams in the paper feed path 20 or near theresist rollers 17A and 17B, as shown in FIG. 5, the rear end of theguide member 22 is lifted and the guide member 22 is rotated to open thepaper feed path 20, whereby jamming handling can be easily performed. Toreplace the belt unit 25, as described above, after opening the guidemember 22, the belt unit 25 is detached upward from the belt unitattaching part 24 and replaced. When the sheet 3 jams near the paperfeed roller 10 or near the paper dust removing rollers 13A and 13B, asshown in FIG. 2, by drawing the discharge tray 8 out of the unit frame7, said jamming can be solved.

As described above, according to the example structure of the invention,the guide member 22 has the guide surface 60 which the tip end portionof the sheet 3 slidably comes into contact with. The guide surface 60 ofthe guide member 22 is curved into a concave shape so that the sheet 3gradually changes its direction as it forwards to the downstream side ofthe conveying direction. By thus configuration, the sheet 3 fed out fromthe resist rollers 17A and 17B turns into a posture that the portionfrom the tip end portion to the nipping position between the resistrollers 17A and 17B gradually curves as it forwards to the downstreamside while the tip end portion is in rubbing contact with the guidesurface 60 of the guide member 22, and a space is formed between thesame and the guide surface 60 due to the elasticity of the sheet 3.Thereby, the sheet 3 can be smoothly curved. When the conveying speed Vrof the sheet 3 by the resist rollers 17A and 17B is higher than theconveying speed Vb on the belt 29, the slack of the sheet 3 is releasedto the space between the sheet and the guide surface 60, so that thesheet 3 can be easily slackened. Therefore, without a great load on thesheet 3 itself, the sheet 3 can be stably conveyed.

The sheet 3 fed out from the guide member 22 comes into contact with theconveying surface 29A of the belt 29 from the diagonally upper side.When the sheet 3 fed out from the guide member 22 comes into parallelcontact with the conveying surface 29A of the belt 29, the sheet 3 mayfloat from the conveying surface 29A of the belt 29, however, accordingto this construction, the sheet 3 is pressed against the conveyingsurface 29A of the belt 29, so that the sheet 3 can be reliably broughtinto close contact with the conveying surface 29A of the belt 29.

With respect to the conveying speed Vb of the sheet 3 on the belt 29,the conveying speed Vr of the sheet 3 by the resist rollers 17A and 17Bis set to Vr>Vb. Thereby, it can be prevented that drawing of the sheet3 between the belt 29 and the resist rollers 17A and 17B makes theconveyance of the sheet 3 unstable and harmfully influences the qualityof an image to be recorded.

The conveying rollers for feeding-out the sheet 3 to the belt 29 sideare resist rollers 17A and 17B that correct the tip end portion of thesheet 3, so that other resist rollers are not necessary, and theconstruction becomes simple.

The contact angle of the tip end portion of the sheet 3 with the guidesurface 60 of the guide member 22 is set to be always not more than 45degrees. When the tip end portion of the sheet 3 is pressed against theguide surface 60 at a great contact angle, the load on the sheet 3increases and the tip end portion of the sheet 3 may be damaged,however, according to the present construction, the tip end portion ofthe sheet 3 comes into contact with the guide surface 60 always at agentle angle, so that the sheet 3 can be prevented from being damaged.

An inner guide member 21 for forming a conveying path for the sheet 3between the same and the guide member 22 is provided, and on this innerguide member 21, a convex guide surface 63 curved into a convex shape soas to face the guide surface 60 is formed, so that the sheet 3 can besmoothly guided. For example, even when the rear end of the sheet 3vibrates in the thickness direction, this vibration can be reduced.

The guide member 22 is made movable to open the paper feed path 20 as aconveying path for the sheet 3, so that jamming of the sheet 3 on theinner side of the guide member 22 can be easily handled.

The feed-out direction D1 of the sheet 3 from the resist rollers 17A and17B is inclined toward the conveying direction of the sheet 3 on thebelt 29 more than the direction orthogonal to the conveying surface 29Aof the belt 29 for the sheet 3 on the belt 29. Thereby, even when theapparatus is downsized, curvature of the sheet 3 in the paper feed path20 can be made small.

By forming a reinforcement edge 62 extending to the side of a surfaceopposite the guide surface 60 on the downstream side end of the guidemember 22, the strength of the end of the guide member 22 can beimproved.

According to the example structure of the laser printer 1, theconveyance accuracy of the sheet 3 is secured by the conveying unit 4including the guide member 22 having the guide surface 60, so thathigh-quality image forming is realized.

In addition, the transfer roller 31 is disposed continuously from thedownstream side of the belt support roller 27 disposed closest to theguide member 22, and the guide member 22 brings the tip end portion ofthe sheet 3 into contact between the support position of the beltsupport roller 27 and the contact position of the transfer roller 31 onthe belt 29. Thereby, it becomes unnecessary to provide a roller or thelike for pressing the sheet 3 against the belt 29 between the beltsupport roller 27 and the transfer roller 31. Therefore, the number ofcomponents can be reduced and downsizing of the apparatus is realized.

In addition, the guide member 22 is disposed so that a part of thisenters between the process cartridge 35 and the belt 29, whereby theapparatus can be downsized. The conveying unit 4 including the resistrollers 17A, 17B, the guide member 22, and the belt 29 is formed so thatit can be drawn out of the main body casing 2, so that maintenance suchas jamming handling and parts replacement can be easily performed.

By forming the conveying path for the sheet 3 into a so-called S shape,the apparatus according the example structure can be made compact.

By applying the example structure of the present invention to a directtandem color laser printer including developing cartridges 44 andphotosensitive drums 42 for the respective colors, the conveyanceaccuracy of the sheet 3 is secured, so that high-quality color imageforming is realized.

<Another Example Structure>

Another example structure will be described with reference to FIG. 1 andFIG. 11. Description of the construction substantially the same as thatof the example structure will be omitted.

FIG. 10 is an enlarged sectional side view of the vicinity of the guidemember, and FIG. 11 is a sectional view along X-X of FIG. 10.

In the example structure, to protrude the width direction of the sheet 3toward the conveying surface 29A of the belt 29, the protrusion ribs 200are used, however, another example structure is different from theexample structure in that a roller 240 serving as a guiding member andthe rotary body is used instead.

Therefore, description of the same points as in the example structurewill be omitted.

On the guide member 22, as shown in FIG. 10 and FIG. 11, a roller 240 asan example of the guiding member, disposed adjacent to the conveyingdirection downstream side of the guide surface 60 so as to face theconveying surface 29A of the belt 29, is rotatably supported.

The roller 240 is supported on a bearing 243 of the guide member 22rotatably via a rotation shaft 240 a so that the peripheral surface 241thereof protrudes more than the ribs 210 at the width direction centralportion of the guide member 22 as shown in FIG. 11.

With this construction, when the sheet 3 is guided by the guide surface60 toward the roller 240, the width direction central portion of thesurface of the sheet 3 opposite the surface facing the conveying surface29A of the belt 29 comes into contact with the peripheral surface 241 ofthe roller 240 and the width direction central portion of the tip endportion of the sheet 3 curves so as to protrude toward the conveyingsurface 29A of the belt 29. When the sheet 3 is thus guided while beingcurved by the peripheral surface 241 of the roller 240, the sheet 3 canbe guided to the conveying surface 29A of the belt 29 in a manner inthat, first, the width direction central portion which was curved andprotruded toward the conveying surface 29A side of the sheet 3 comesinto contact with the conveying surface 29A of the belt 29, and whileconveyed, this contact area gradually spreads to both ends in the widthdirection of the sheet 3 and air between the conveying surface 29A ofthe belt 29 and the sheet 3 is released to both end sides.

When the sheet 3 comes into contact with the roller 240, the rollerrotates to release the force of this contact, so that the conveyanceresistance to the sheet 3 can be suppressed. As a result, the sheet 3can be prevented from jamming near the roller 240.

<Still Another Example Structure>

Next, another example structure will be described with reference to FIG.12. The construction according to still another example structure issubstantially the same as that of the example structure except for theshape of the guide member 65. Description of the constructionsubstantially the same as that of the example structure will be omitted.

The guide member 65 o has a plate-shaped part 66 formed along the paperfeed path 20, and on the lower surface of this plate-shaped part 66, aguide surface 67 which the tip end portion of the sheet 3 fed out fromthe resist rollers 17A and 17B can come into rubbing contact with isformed. The guide surface 67 is curved into a concave shape so that itgradually changes in direction from the upper stream side toward thedownstream side as in the example structure, and the guide surfacegradually separates from the conveying surface 29A of the belt 29 andthen gradually approaches the conveying surface 29A of the belt 29 in adirection orthogonal to the conveying surface 29A of the belt 29. InFIG. 12, the point G on the plate-shaped part 66 indicates a position atwhich the tip end portion of the sheet 3 fed out from the resist rollers17A and 17B comes into contact with the plate-shaped part 66 for thefirst time, and corresponds to the upstream side end of the guidesurface 67. On the plate-shaped part 66, an extended guide surface 68extended to the upstream side from the upstream side end of the guidesurface 67 is formed, and this extended guide surface 68 is formed toextend to the vicinity of the nipping position between the resistrollers 17A and 17B. The extended guide surface 68 is formed so as to beinclined to the diagonally lower rear side smoothly continuously to theguide surface 67, and can be rubbing-contacted by a part of the sheet 3(a part other than the tip end portion).

Furthermore, adjacent to the downstream side end of the guide surface27, protrusion ribs 200 similar to those of the example structure areprovided, and ribs 220 are provided so as to face the guide member 25.

On the guide member 65, the extended guide surface 68 extended to theupstream side more than the guide surface 67 that the tip end portion ofthe sheet 3 comes into contact with is formed up to the vicinity of theresist rollers 17A and 17B, so that, for example, the rear end of thesheet 3 can be prevented from springing up and vibrating when it passesthrough the resist rollers 17A and 17B. Therefore, the sheet 3 can bestably conveyed, and the quality of an image to be recorded can besecured.

In the same drawing, the position of the sheet 3 when the tip endportion of the sheet 3 comes into contact with the point G is shown bythe line P1, and examples of the angles between the sheet 3 at thecontact point at which the tip end portion of the sheet 3 is in contactwith the guide surface 67 and the normal of the guide surface 67 whenthe sheet 3 moves toward the downstream side to the positions of P2, P3. . . P8 in order are shown. As described above, the contact angle E ofthe tip end portion of the sheet 3 with the guide surface 67 can becalculated from “E=90°−C” provided that the angle between the tip endportion of the sheet 3 and the normal of the guide surface 67 at thecontact point. Thereby, when the sheet 3 is at the position of P1,E=90°−64°=26°, and as the sheet 3 moves to P2, P3 . . . , the value of Egently changes to 32°, 34°, 35°, 39°, 40°, 40°, and 32° in order. Thepositions of P1 through P8 indicate positions on the assumption that thesheet 3 has sufficiently high rigidity, and in actuality, according tothe level of elasticity of the sheet 3, the sheet 3 curves outward, sothat the value of the contact angle E becomes smaller than theabove-described values.

As described above, the contact angle E of the tip end portion of thesheet 3 with the guide surface 67 is set to be always not more than 45degrees (40 degrees or less). Thereby, the sheet 3 comes into contactwith the guide surface 67 always at a gentle angle, so that the load onthe sheet 3 can be suppressed. The sheet 3 is most difficult to becurved when its tip end portion comes into contact with the guide member65 for the first time, however, as described above, the contact angle Eof the tip end portion of the sheet 3 is set to be a small value nearthe point G on the guide surface 67, so that curving can be startedwithout great loads on the sheet 3 and the resist rollers 17A and 17B.

<Still Yet Another Example Structure>

Next, still yet another example structure will be described withreference to FIG. 13. In the following description, description of theconstruction substantially the same as in the example structure will beomitted.

On the downstream side of a guide member 70 provided in an image formingapparatus main body having a guide surface 71, a protrusion rib 250 asan example of the guiding member is provided so as to face the conveyingsurface 29A of the belt 29 on the lower end of a case 74 of a developingcartridge 73 provided detachably from the process cartridge 72. That is,the protrusion rib 250 protrudes in the width direction central portionof the tip end portion of a sheet 3 fed from the guide 70 toward theconveying surface 29A of the belt 29, and feeds the tip end portion ofthe sheet 3 to the conveying surface 29A of the belt 29.

With this construction, when the sheet 3 is guided by the guide surface71 to the protrusion rib 250, the width direction central portion (inthe recording sheet conveying direction) of the surface of the sheet 3opposite the surface facing the conveying surface 29A of the belt 29comes into contact with the protrusion rib, and the width directioncentral portion of the tip end portion of the sheet 3 curves so as toprotrude toward the conveying surface 29A of the belt 29. When the sheet3 is thus guided while curved by the protrusion rib 250, the sheet 3 canbe guided to the conveying surface 29A of the belt 29 in a manner inthat, first, the width direction central portion of the sheet 3 curvedand protruded toward the conveying surface side comes into contact withthe conveying surface 29A of the belt 29, the contact area graduallyspreads to both end sides of the width direction of the sheet 3 whileconveyed, and air between the conveying surface 29A of the belt 29 andthe sheet 3 is released to both end sides.

The protrusion rib 250 is fixed to the developing cartridge 73 which isdetachably attached to the image forming apparatus main body, and on theother hand, the guide member 70 is provided in the image formingapparatus main body separately from the protrusion rib 250, so that whenthe sheet 3 jams between the protrusion rib 250 and the guide member 70,the sheet 3 can be easily removed by detaching the developing cartridge73 from the image forming apparatus main body.

<Further Still Yet Another Example Structure>

Next, further still yet another example structure will be described withreference to FIG. 14 through FIG. 16. In FIG. 14 and FIG. 15, aprotrusion rib 270 serving as the guiding member and a rib 280 servingas the auxiliary guiding member are not shown. In the followingdescription, description of the construction substantially the same asin the example structure will be omitted.

The laser printer 80 is a direct tandem color laser printer includingfour photosensitive drums 116 corresponding to the colors of black,cyan, magenta, and yellow, respectively. The laser printer 80 includes,in a main body casing 81, a sheet conveying device 83 for conveying asheet 82 as a recording sheet and an image forming unit 84 for formingan image on the sheet 82 conveyed by the sheet conveying device 83. Inthe following description, the right side of FIG. 15 is defined as thefront side.

On the lower side of the main body casing 81, a paper feeding tray 86which can be drawn out forward is provided, and sheets 82 stackedtherein are supplied to resist rollers 91A and 91B of the sheetconveying device 83 by a pickup roller 87, a paper feed roller 88, aseparating pad 89, and a pair of paper dust removing rollers 90.

The sheet conveying device 83 includes the pair of resist rollers 91Aand 91B, a guide member 92, and a belt unit 94. A sheet 82 fed out fromthe resist rollers 91A and 91B passes through a paper feed path 95formed between the guide member 92 and an inner guide member 93 and isconveyed onto a belt 96 of a belt unit 94. The belt unit 94 includes apair of front and rear belt support rollers 97, the inner guide member93, the belt 96, a cleaning roller 98, and transfer rollers 99, etc.,and the entirety is detachable from the main body casing 81. While thesheet 82 is conveyed rearward on the belt 96, transfer of images in therespective colors is performed by the photosensitive drums 116 of theimage forming unit 84 and the transfer rollers 99 of the belt unit 94.Then, the sheet 82 passes through a fixing device 100 disposed rear ofthe belt unit 94 and is ejected onto the discharge tray 102 on the uppersurface of the main body casing 81 by an eject roller 101.

On the upper side of the main body casing 81, a scanner 104 as anexample of the exposing unit which emits a laser beam L onto therespective photosensitive drums 116 is provided, and between the scanner104 and the belt 96, an image forming unit 84 is housed. On the frontface of the main body casing 81, a front cover 105 that can open andclose is provided, and by opening this front cover 105, the imageforming unit 84 becomes able to be drawn out forward of the main bodycasing 81. The image forming unit 84 includes a substantially box-shapedframe 107. On the frame 107, four cartridge attaching parts 108 openedupward are provided in line in the front and rear direction, and to therespective cartridge attaching parts 108, four developing cartridges 109corresponding to the respective colors are detachably attached. Eachdeveloping cartridge 109 includes a toner containing chamber 111, asupply roller 112, a developing roller 113, and a layer thicknessrestricting blade 114 in the case 110. On the frame 107, on the lowerside of each cartridge attaching part 108, a photosensitive drum 116disposed so as to face the developing roller 113 and the transferrollers 99 is held, and furthermore, around the photosensitive drum, ascorotron charger 117 and a cleaning brush 118 are held. On the bottomsurface of the front side of the frame 107, the above-described guidemember 92 having a guide surface 119 curved into a concave shape isformed integrally. One resist roller 91B is held on the front end side(upstream side) of the guide member 92. On the main body casing 81 side,an inner guide member 93 including a convex guide surface 120 facing theguide surface 119 and the other resist roller 91A are provided.

At the width direction central portion of the guide member 92, aprotrusion rib 270 as an example of the guiding member is projectedlyfixed so as to come into contact with the width direction centralportion of the sheet 3.

To the inner guide member 120, a rib 280 as an example of the auxiliaryguiding member that faces the guide member 92 and protrudes is fixed.

When the sheet 82 is guided by the guide surface 119 toward theprotrusion rib 270, the width direction central portion (in therecording sheet conveying direction) of the surface of the sheet 82opposite the surface facing the conveying surface of the belt 96 comesinto contact with the protrusion rib, and the width direction centralportion of the tip end portion of the sheet 82 curves so as to protrudetoward the conveying surface of the belt 96. When the sheet 82 is guidedwhile thus curved by the protrusion rib 270, the sheet 82 can be guidedto the conveying surface in a manner that, first, the width directioncentral portion curved and protruded toward the conveying surface sideof the belt 96 of the sheet 82 comes into contact with the conveyingsurface, and the contact area gradually spreads to both end sides in thewidth direction of the sheet 82 while conveyed, and air between theconveying surface and the sheet 82 is released to both end sides.

The rib 280 (inner guide) is provided integrally with the belt 96 by ajoint mechanism that is not shown, so that its positional accuracy withrespect to the belt 96 increases, and the sheet 82 is more reliablyguided to the conveying surface of the belt 96.

By detaching the belt 96 from the image forming apparatus main body, asheet 82 jamming at the rib 280 can be more easily removed.

The photosensitive drum 116 and the protrusion rib 270 are integrallyprovided via the frame 107, so that the positional accuracy of theprotrusion rib 270 with respect to the photosensitive drum 116 isimproved, and the sheet 82 can be accurately guided to the belt 96facing the photosensitive drum 116.

By detaching the frame 107 from the image forming apparatus main body,the sheet 82 jamming at the protrusion rib 270 can be easily removed.

In the laser printer 80, by drawing the image forming unit 84 out of themain body casing 81, each developing cartridge 109 can be exchanged. Bydetaching the drawn-out image forming unit 84 from the main body casing81, the upper surface of the belt 96 and the paper feed path 95 areopened, so that jamming handling or parts replacement of the belt 96,etc., can be easily performed.

<Another Example Structure>

Next, another example structure will be described with reference to FIG.17. In the following description, description of the constructionsubstantially the same as further still yet another example structurewill be omitted.

In the laser printer 80, a sheet 82 having a surface on which an imagewas formed is fed to an eject roller 101, and is fed into a re-conveyingmechanism 290 by reverse-rotation of the eject roller 101. The sheet 82fed into the re-conveying mechanism 290 is conveyed again to the resistrollers 91A and 91B in a reversed state (the dashed line in FIG. 17indicates a conveying path of the sheet since the time when the ejectroller 101 rotates in reverse).

With this construction, a surface of the sheet 82 conveyed by there-conveying mechanism 290, opposite the printed surface, comes intocontact with the protrusion rib 270, so that stain of the printedsurface and scattering of the toner from this surface can be prevented.

<Still Another Example Structure>

Next, still another example structure will be described with referenceto FIG. 18 through FIG. 22. In the following description, description ofthe construction substantially the same as in further still yet anotherexample structure will be omitted.

A guiding member 700 is different from the protrusion rib 270 in that itis supported so as to be slidable between a position projecting towardthe surface of the guided sheet 82 and a position retracted from thesurface of the sheet 82. A guide surface 920 shown in FIG. 18 is fixedto a frame 107.

To the frame 107, a support member 800 which supports the guiding member700 in a manner enabling it to slide between the position projectingtoward the sheet 82 guided from the guide surface 920 and the positionretracted from the sheet 82 is fixed. Herein, the “retracted position”is positioned at a diagonally upper side from the front side withrespect to the “projecting position.” Hereinafter, the sliding movementdirection of the guiding member 700 is referred to as “slidingdirection,” and the side of the “projecting position” in the slidingdirection is referred to as “projection side” and the side of the“retracted position” is referred to as “retracting side.”

[Guiding Member 700]

The guiding member 700 integrally has, as shown in FIG. 20 and FIG. 21,a first side portion 740 and a second side portion 750 disposed on bothsides in the width direction, respectively, and a central portion 730disposed at a width direction center.

The central portion 730 is a member with substantially a trapezoid shapein a side view, integrally having a sliding portion 710 disposed on thelower side (projection side) and a receiving portion 730 disposed on theupper side (retracting side) as shown in FIG. 18 and FIG. 19. Betweenthe sliding portion 710 and the receiving portion 730 of the centralportion 730, an exposure hole 700 c for exposing the support member 800is formed.

The sliding portion 710 integrally has a claw 711, and this claw isfitted in a sliding hole 835 (see FIG. 22) formed in a support member800 described later so as to be slidable in the sliding direction.

The receiving portion 730 fits a compression coil spring 810 (see FIG.22) serving as an urging unit and is urged to the projection side towardthe sheet 82 in the sliding direction. Herein, as described later, whenthe sheet 82 guided from the guide surface 920 comes into contact withthe guiding member 700, the guiding member 700 is pushed to theretracting side by the elasticity (elastic force) of the sheet 82.

On a lower side of the central portion 730 on a side facing theconveying surface of the belt 96A 96, a first surface 700 a forprotruding the width direction central portion of the sheet 82 towardthe conveying surface of the belt is formed, and continuously from theupper end of the lower side, a second surface 700 b is formed.

The first surface 700 a is a plane inclined so as to extend upwardtoward the front side with respect to the conveying surface of the beltextending substantially horizontally as shown in FIG. 18 and FIG. 19.

The second surface 700 b extends while inclined upward toward the frontside, and the upward inclination is formed so as to form a plane steeperthan the first surface.

In the first side portion 740, a cylindrical sliding hole 741 extendingin the sliding direction is formed. This cylindrical sliding hole 41 issupported so as to be slidable with a first restricting member 810 (seeFIG. 22) of a support member 800 described later in the slidingdirection.

In the second side portion 750, a cylindrical sliding hole 751 is formedsimilarly to the first side portion 740, and this cylindrical slidinghole 752 is supported so as to be slidable with a second restrictingmember 820 of the support member 800 described later in the slidingdirection.

[Support Member 800]

The support member 800 integrally includes, as shown in FIG. 20 and FIG.22, auxiliary guiding surfaces 870 disposed on both sides in the widthdirection, the first restricting member 810 and the second restrictingmember 820 disposed more inward in the width direction than theauxiliary guiding surfaces 870, and a third restricting member 830disposed more inward in the width direction than the first restrictingmember 810 and the second restricting member 820.

The auxiliary guiding surfaces 870 are formed always at positionsretracted from the conveyed sheet 82 more than the first surface 700 aof the leading rib 700, and guides both sides in the width direction ofthe sheet 82 guided from the guide surface 920 further downward.

The second restricting member 820 and the third restricting member 830are rod-shaped members which extend in the sliding direction and havesubstantially rectangular sections orthogonal to the sliding direction.

By fitting with the cylindrical sliding hole 741 and the cylindricalsliding hole 751, respectively, the second restricting member 820 andthe third restricting member 830 restrict movements in all directionsorthogonal to the sliding direction of the guiding member 700 via theholes, and support the guiding member 700 slidably in the slidingdirection.

In the third restricting member 830, a sliding hole 835 is formed on theprojection side, and on the retracting side, a spring seat 840 isformed, and between the sliding hole 835 and the spring seat 840, aspring hole 850 is formed.

As shown in FIG. 18, FIG. 19, and FIG. 22, the sliding hole 835 supportsthe guiding member 700 slidably in the sliding direction, by fitting theclaw 711 of the sliding portion 710, and has a projection siderestricting surface 835 a which restricts movements of the claw 71 tothe projection side and a retracting side restricting surface 835 bwhich restricts movements of the claw 71 to the retracting side.

On the spring seat 840, as shown in FIG. 18 and FIG. 19, one end of the(compression) coil spring 810 is supported. Herein, the compression coilspring 810 urges the guiding member 700 to the projection side, and theurging force of the coil spring 810 is described herein.

That is, the urging force of the compression coil spring 810 is at alevel which does not greatly displace itself even when pushed toward theretracting side via the guiding member 700 by the elasticity (elasticforce) of the sheet 82 when a plain paper being low in elasticity as thesheet 82 comes into contact with the guiding member 700.

On the other hand, the urging force of the compression coil spring 810is limited to the degree at which, when a thick paper with elasticityhigher than that of a plain paper as the sheet 82 comes into contactwith the guiding member 700, the urging force of the compression coilspring is overcome by the pressing force of the sheet 82 when pushed bythe elasticity (elastic force) of the sheet 82 to the retracting side,and the compression coil spring is compressed to move the guiding member700 to the above-described “retracted position.”

The spring hole 850 is a hole in a rectangular shape extending in thesliding direction, and holds the spring 810, apart of the spring seat820, and apart of the receiving portion 730, and restricts theirdisplacements in the width direction.

[Guide Surface 920]

In the guide surface 920, as shown in FIG. 20, on a width directioncentral portion thereof, a U-shaped notched portion is formed so as tosandwich the retracting side of the sliding direction of the guidingmember 700.

The tip end 923 of the guide surface 920 is formed so as to be inclineddownward toward the rear side as shown in FIG. 18, and extends so as tocross the second surface 700 b of the guiding member 700 in a side viewwhen the guiding member 700 is at the projecting position of the slidingdirection.

When a plain paper being low in elasticity is guided as the sheet 82from the guide surface 920, the tip end portion of the sheet 82 comesinto contact with the second surface 700 b, the corner 700 d on theboundary between the second surface 700 b and the first surface 700 a,and the first surface 700 a of the guiding member in order as it isconveyed.

When the sheet 82 is in contact with the first surface 700 a, the firstsurface 700 a is pushed by a force F in a direction substantiallyorthogonal to the first surface 700 a due to the elasticity of the sheet82 as shown in FIG. 18. Herein, the force F has a component f directedtoward the retracting side of the sliding direction, so that the guidingmember 700 is pushed toward the retracting side. However, the guidingmember 700 is not greatly displaced by the urging force of the coilspring 810 even when pushed by the elasticity of the plain paper.

Namely, when a plain paper is thus used, the guiding member 700 isalways at the “projecting position,” so that the plain paper comes intocontact with the conveying surface of the belt 96A while its widthdirection central portion is curved to project toward the conveyingsurface of the belt due to the contact with the guiding member 700.

On the other hand, when a thick paper being higher in elasticity thanthat of the plain paper is guided as the sheet 82 by the guide surface920, as in the case of the plain paper, the first surface 700 a of theguiding member 700 is subjected to a force F′ directed substantiallyorthogonal to the first surface 700 a from the sheet 82 due to theelasticity of the sheet 82. Then, the force F′ has a component f′directed toward the retracting side of the sliding direction, however,the elasticity of the thick paper is higher than that of the plainpaper, so that the strength of the F′ is greater than F, and as aresult, f′ is also greater than f. Therefore, due to the force f∝, theguiding member 700 is displaced from the “projecting position” to the“retracted position” shown in FIG. 19 against the force of the coilspring 810.

When a thick paper is thus used, the sheet 82 comes into contact withthe conveying surface of the belt 96A without being greatly curved bythe guiding member 700.

When a plain paper being low in elasticity is used as the sheet 82, airis trapped between the sheet 82 and the conveying surface of the beltand easily lowers the image quality, so that the widthwise centralportion of the sheet 82 is curved so as to protrude toward the conveyingsurface of the belt and the sheet is brought into contact with the beltfrom this central portion.

However, when a thick paper with elasticity higher than that of theplain paper is used, due to the high elasticity of the sheet 82, inparticular, the problem of air being trapped between the sheet 82 andthe conveying surface of the belt does not occur. On the contrary, whena sheet 82 with such high elasticity is forcibly curved by a forcestronger than a predetermined strength, this curve may be hardlyrestored. That is, when the sheet 82 is fed to the conveying surface ofthe belt 96A in such a greatly curved state, even after the sheet 82 isplaced on the conveying surface and conveyed to a transfer position, itremains curved, and a developing image may not be normally transferredon the sheet 82.

Therefore, when a plain paper is guided as the sheet 82 from the guidesurface 920, the guiding member 700 sufficiently curves a centralportion of the sheet 82 toward the conveying surface of the belt andthen brings it into contact with the conveying surface of the belt, andon the other hand, when a thick paper is guided as the sheet 82 from theguide surface 920, the guiding member 700 shifts to the “retractedposition” so as not to greatly curve the sheet.

Thus, when a plain paper is used, air trapped between the conveyingsurface of the belt and the sheet 82 can be prevented, and when a thickpaper is used, the thick paper can be prevented from being excessivelycurved and lowering the image quality.

A plain paper is used as a sheet being lower in elasticity than that ofthe thick paper, however, instead of the plain paper, a thin paper withelasticity lower than that of the plain paper may also be used.

<Still Yet Another Example Structure>

Next, still yet another example structure will be described withreference to FIG. 23 through FIG. 27. In the following description,description of the construction substantially the same as in anotherexample structure will be omitted.

A protrusion rib 900 is different from the protrusion rib 270 in thatthe protrusion rib 900 is supported in a manner enabling it to swingbetween a position projecting toward a guided sheet 82 and a positionretracted from the sheet 82, and this swing is controlled by a CPU.

The laser printer 80 comprises, as shown in FIG. 23, a sensor A which isdisposed on the more upstream side in the conveying direction of thesheet 82 than the resist rollers 91A and 91B and detects passing of thetip end portion of the sheet 82, and a sensor B (see FIGS. 24A, 24B)which is disposed on the more downstream side in the conveying directionthan the protrusion rib 900 and detects passing of the rear end of thesheet 82.

The protrusion rib 900 has, as shown in FIGS. 24A, 24B and FIGS. 25A,25B, a guiding surface 900 a which is shaped into a wedge shape in aside view and guides a sheet 82 conveyed from the guide surface 119 tothe conveying surface of the belt 96A, and a swing shaft 900 b supportedon the frame 107 on the upstream side in the conveying direction in amanner enabling it to swing.

With this construction, as shown in FIG. 24A and FIG. 24B, theprotrusion rib 900 can swing around the swing shaft 900 b between the“projecting position” at which the guiding surface 900 a projects towardthe surface of the sheet 82 conveyed from the guide surface 119 and the“retracted position” at which the guiding surface 900 a is retractedfrom the surface of the sheet 82. Herein, as shown in FIGS. 24A, 24B, inthe swinging direction, the side of the “projecting position” isreferred to as “projection side,” and the side of the “retractedposition” is referred to as “retracting side.”

To the opposite side of the guiding surface 900 b of the protrusion rib900, as shown in FIGS. 25A, 25B, the other end portion of a tension coilspring 950 having one end fixed to the frame 107 is fixed. With thisconstruction, the protrusion rib 900 is urged toward the “retractingside” by the tension coil spring 950.

With the opposite side of the guiding surface 900 a of the protrusionrib 900, a switching member 960 for switching the position of theprotrusion rib 900 to either the “projecting position” or the “retractedposition” is in contact.

The switching member 960 has a cam 910 that comes into contact with theopposite surface of the guiding surface 900 a of the protrusion rib 900,a tension coil spring 930 disposed on one side in the width direction ofthe cam 910, and a solenoid 970 disposed on the other end side in thewidth direction of the cam 910.

The cam 910 is supported on the frame 107 slidably movably in the widthdirection, and has a projecting surface 910 b projecting toward theprojection side on the other side in the width direction, a retractedsurface 910 a retracted to the retracting side more than the projectingsurface 910 b on one side in the width direction, and an inclinedsurface 910 c which extends in an inclined manner with respect to thewidth direction and joins the projecting surface 910 b and the retractedsurface 910 a.

The tension coil spring 930 has one end fixed to the frame 107 and theother end fixed to the cam 910, and urges the cam 910 toward one side inthe width direction.

The solenoid 970 is fixed to the cam 910, and when the solenoid is ON,it urges the cam 910 toward the other side in the width direction, andwhen it is OFF, the solenoid releases this urging force. Herein, the ONstate of the solenoid 970 means a state that power is supplied to thesolenoid 970, and the OFF state of the solenoid 970 means a state thatno power is supplied to the solenoid 970. With this construction of thesolenoid, when the solenoid 970 is OFF, the cam 910 is supported at aposition at which the projecting surface 910 b comes into contact withthe protrusion rib 900 by the urging force of the tension coil spring930 as shown in FIG. 25A. On the other hand, when the solenoid 970 isON, the urging force of the solenoid 970 overcomes the urging forces ofthe tension coil spring 950 and the tension coil spring 970, whereby thecam 910 slides and moves to the position at which the retracted surface900 a comes into contact with the protrusion rib 900 as shown in FIG.25B.

The laser printer 80 further includes, as shown in FIG. 26, a controlpart 990 for generally controlling the entirety of the apparatus, and anoperating part 980 having operation keys (not shown), etc., that a usercan operate. To the control part 990, the operating part 980, the sensorA, the sensor B, the solenoid 970, the image forming unit 94, the sheetconveying device 83, and the operating part 980, etc., are electricallyconnected.

The control part 990 includes a CPU for performing various arithmeticoperations, a ROM for storing various programs to be executed by theCPU, and a RAM to be used as a work area when a program is executed bythe CPU. The control part 990 realizes image forming through the imageforming unit 94 by executing various programs by the CPU.

The operating part 980 accepts a selection as to whether the sheet 82 onwhich an image will be formed is a plain paper or a thick paperaccording to an input operation performed on operation keys by a user.Herein, the thick paper means a sheet being higher in elasticity thanthat of the plain paper. The result of selection between the plain paperand the thick paper is transmitted as an electrical signal to thecontrol part 990.

When the sensor A detects passing of the tip end portion of the sheet82, the result of this detection is transmitted as an electrical signalto the control part 990, and in the control part 990, a sensor A passingflag is turned ON.

When the sensor B detects passing of the rear end of the sheet, theresult of this detection is transmitted as an electrical signal to thecontrol part 990, and in the control part 990, the sensor B passing flagis turned ON.

This solenoid control processing is executed by the CPU of the controlpart 990, and is started every predetermined time (for example, every0.1 second) in a state that the power source of the laser printer 80 isON.

When this solenoid control processing is started, the CPU of the controlpart 990 judges whether the tip end portion of the sheet 82 has passedthrough the sensor A (S1000).

Then, at S1000, when it is judged that the tip end portion of the sheet82 has not passed through the sensor A (No at S1000), this solenoidcontrol processing is ended.

On the other hand, when it is judged that the tip end portion of thesheet 82 has passed (the sensor A flag is ON) (Yes at S1000), theprocess shifts to S1010.

When the process shifts to S1010, the sensor A passing flag is turnedOFF, and when it is judged that a thick paper is not set as the sheet 82to be used in the operating part 980, that is, when it is judged that anormal paper has been set (No at S1010), this solenoid controlprocessing is ended.

On the other hand, when it is judged that a thick paper is used (Yes atS1010), the process shifts to S1030.

When the process shifts to S1030, the solenoid 970 is turned ON.

When the solenoid 970 is thus turned ON, the cam 910 is urged toward theother side in the width direction by the solenoid 970, and slides andmoves from the position shown in FIG. 25B at which the projectingsurface 910 b comes into contact with the protrusion rib 900 to theposition shown in FIG. 25B at which the retracted surface 910 a facesthe protrusion rib 900.

In this sliding movement, the protrusion rib 900 is urged to theretracting side by the tension spring 950 and moves to the retractedposition while rubbing against the inclined surface 910 c. As a result,as shown in FIG. 24B, the guiding surface 900 a is at the positionretracted from the surface of the conveyed sheet 82 (thick paper), sothat the sheet 82 that is a thick paper is guided to the conveyingsurface of the belt 96A without being greatly curved.

When the processing of S1030 is thus finished, the process shifts toS1050 and waits until the rear end of the sheet 82 is detected by thesensor B, and when it is judged that the rear end of the sheet 82 wasdetected by the sensor B (the sensor B flag is ON) (Yes at S1050), theprocess shifts to S1070.

When the process shifts to S1070, the sensor B passing flag is turnedOFF, and the solenoid 970 is turned OFF.

When the solenoid is thus turned OFF, the urging force of the solenoid970 to urge the cam 910 is released, and the cam 910 is urged to oneside in the width direction by the tension coil spring 930. Thus, thecam 910 slides and moves from the position shown in FIG. 25B at whichthe retracted surface 910 a comes into contact with the protrusion rib900 to the position shown in FIG. 25B at which the projecting surface910 b comes into contact with the protrusion rib 900.

In this sliding movement, the protrusion rib 900 moves to the projectingposition while rubbing against the inclined surface 910 c as shown inFIG. 24A.

When a plain paper being low in elasticity is used as the sheet 82, airis trapped between the sheet 82 and the conveying surface of the belt96A and easily lowers the image quality, so that the widthwise centralportion of the sheet 82 is curved so as to protrude toward the conveyingsurface of the belt 96A and the sheet is brought into contact with theconveying surface of the belt 96A from the central portion.

However, when a thick paper being higher in elasticity than that of aplain paper is used as the sheet 82, due to the high elasticity of thesheet 82, the problem in particular of air being trapped between thesheet 82 and the conveying surface of the belt does not occur. On thecontrary, if the sheet 82 being of such high elasticity is forciblycurved by a force stronger than a predetermined strength, the curvaturemay be hardly restored. That is, if the sheet 82 is fed to the conveyingsurface of the belt while remaining greatly curved, even after the sheet82 is conveyed on the conveying surface to the transfer position, it isleft curved, and a developing image may not be normally transferred onthe sheet 82.

Therefore, when a plain paper is guided as the sheet 82 from the guidesurface 920, the guiding member 700 sufficiently curves the centralportion of the sheet 82 toward the conveying surface of the belt andthen brings it into contact with the conveying surface of the belt, andon the other hand, when a thick paper is guided as the sheet 82 from theguide surface 920, the guiding member 700 is displaced to the “retractedposition” so as not to greatly curve the sheet.

Thus, when a plain paper is used, air trapped between the conveyingsurface of the belt and the sheet 82 can be prevented, and when a thickpaper is used, the thick paper can be prevented from being excessivelycurved and causing a lowering in image quality.

Setting of a thick paper or a plain paper is made through the operatingpart 980, however, it is also possible to detect whether the sheet is athick paper or a plain paper by a sensor.

<Further Still Yet Another Example Structure>

Next, further yet still another example structure will be described withreference to FIG. 28 and FIG. 29. In the following description,description of the construction substantially the same as in still yetanother example structure will be omitted.

The still yet another example structure is constructed so that theprotrusion rib 900 is at the “projecting position” when a plain paper isused as the sheet 82, and is at the “retracted position” when a thickpaper is used as the sheet 82.

Instead of said construction, the protrusion rib 900 is positioned atthe “retracted position” when an image is formed on a first surface ofthe sheet 82, and is positioned at the “projecting position” when animage is formed on a second surface of the sheet 82.

Herein, the first surface of the sheet 82 means either of the front orback surface of the sheet 82 in a state that an image is formed onneither the front nor back surface, and the second surface of the sheet82 means a surface opposite the first surface in a state that an imageis formed on the first surface.

In the mechanical construction, the re-conveying mechanism 290 isprovided with a sensor C that detects the tip end portion of the sheet82. The sensor C is electrically connected to the control part 990, andwhen passing of the tip end portion of the sheet 82 is detected by thesensor C, the result of this detection is transmitted to the controlpart 990, and a sensor C passing flag is turned ON in the control part990.

This solenoid control processing is started every predetermined time(for example, every 0.1 second) in a state that the power source of thelaser printer 80 is ON.

When this solenoid control processing is started, the CPU of the controlpart 990 judges whether the tip end portion of the sheet 82 has passedthrough the sensor A (S2000).

Then, at S2000, when it is judged that the tip end portion of the sheet82 has not passed through the sensor A (No at S2000), this solenoidcontrol processing is ended.

On the other hand, when it is judged that the tip end portion of thesheet 82 has passed through the sensor A (the sensor A passing flag isON) (Yes at S2000), the process shifts to S2010.

When the process shifts to S2010, the sensor A passing flag is turnedOFF, and when it is judged that the tip end portion of the sheet 82 haspassed through the sensor C (sensor C passing flag is ON), that is, itis judged that an image has already been formed on the first surface ofthe sheet 82 and the sheet 82 is being conveyed for image forming on thesecond surface (Yes at S2010), the sensor C passing flag is turned OFFand this solenoid control processing is ended.

On the other hand, when it is judged that the tip end portion of thesheet 82 has not passed through the sensor C, that is, it is judged thatthe sheet 82 is being conveyed for image forming on the first surface(No at S2010), the process shifts to S2030.

When the process shifts to S2030, the solenoid 970 is turned ON.

When the solenoid 970 is thus turned ON, the protrusion rib 900 movesfrom the “projecting position” to the “retracted position.” As a result,when an image is formed on the first surface of the sheet 82, theguiding surface 900 a is at the position retracted from the surface ofthe conveyed sheet 82, so that the sheet 82 is guided to the conveyingsurface of the belt 96A without being greatly curved.

When the processing of S2030 is thus finished, the process shifts toS2050 and waits until the sensor B detects the rear end of the sheet 82,and when it is judged that the rear end of the sheet 82 was detected bythe sensor B (the sensor B passing flag is ON) (Yes at S2050), theprocess shifts to S2070.

When the process shifts to S2070, the sensor B passing flag is turnedOFF, and the solenoid 970 is turned OFF.

When the solenoid is thus turned OFF, the protrusion rib 900 moves fromthe “retracted position” to the “projecting position.” As a result, whenan image is formed on the second surface of the sheet 82, the centralportion of the sheet 82 is sufficiently curved so as to protrude towardthe conveying surface of the belt 96A, and is guided toward theconveying surface of the belt 96A.

When the sheet 82 is guided to the conveying surface of the belt 96A forimage forming on the first surface, an image has not been formed yet, sothat the flatness of the surface of the sheet 82 is high, and thepossibility of air being trapped between the conveying surface of thebelt 96A and the surface of the sheet 82 may be slight. In this case, inimage forming on the first surface, to avoid burden on the sheet 82, itmay be desirable to avoid the above-described curving of the centralportion of the sheet 82.

On the other hand, when the sheet 82 is guided to the conveying surfaceof the belt 96A for image forming on the second surface, an image hasalready been formed on the first surface, so that the flatness of thesurface of the sheet 82 has lowered, so that it is highly possible thatair is trapped between the conveying surface of the belt 96A and thesheet 82. Therefore, in the case of image forming on the second surfaceof the sheet 82, before the sheet 82 comes into contact with theconveying surface of the belt 96A, it is desirable that the centralportion of the sheet 82 is curved so as to protrude toward the conveyingsurface of the belt 96 as described above.

In the case of image forming on the first surface of the sheet 82, theprotrusion rib 900 is at the “retracted position,” so that the sheet 82is not greatly curved. As a result, an undesirable burden on the sheet82 can be avoided.

On the other hand, in the case of image forming on the second surface ofthe sheet 82, the protrusion rib 900 is on the “projecting position,” sothat the central portion of the sheet 82 is sufficiently curved so as toprotrude toward the conveying surface of the belt 96A, and therefore,air being trapped between the conveying surface of the belt 96A and thesurface of the sheet 82 is prevented.

According to the above-described example structures, when a recordingsheet is guided by the guiding member while being curved, the recordingsheet can be guided to the belt so that, first, the width directioncentral portion that was curved and protruded toward the conveyingsurface side of the recording sheet comes into contact with theconveying surface of the belt, and the contact area gradually spreads toboth end sides in the width direction of the recording sheet while beingconveyed, and air between the conveying surface of the belt and therecording sheet is expelled to both end sides.

In addition, the guiding member for leading the tip end portion of therecording sheet toward the conveying surface of the belt is in contactwith a surface of the recording sheet opposite a surface facing theconveying surface of the belt, so that the tip end portion of therecording sheet is restricted from warping in a direction of separatingfrom the conveying surface of the belt, and is brought into contact withthe conveying surface of the belt more reliably than in the constructiondescribed in Patent Document 1. As a result, it can be prevented thatair is trapped between the tip end portion of the recording sheet andthe conveying surface of the belt and lowers the quality of an imageformed on the recording sheet surface.

According to the above-described example structures, the tip end portionof the recording sheet is fed to the guide member recessed into aconcave shape by the conveying rollers, so that the tip end portion ofthe recording sheet becomes difficult to separate from the guide memberwhile being conveyed to reach the downstream side of the guide member,and the tip end portion of the recording sheet can be more reliablyguided to the guiding member.

According to the above-described example structures, the recording sheetbends between the belt and the conveying rollers, and a surface of therecording sheet opposite a surface facing the conveying surface of thebelt becomes easier to more reliably come into contact with theconveying surface of the belt due to a guide member recessed into aconcave shape. As a result, by more reliably guiding this surface by theguiding member, the width direction central portion of the surface inthe recording sheet conveying direction is more reliably protrudedtoward the conveying surface of the belt, and it can be more reliablyprevented that air is trapped between the surface and the conveyingsurface of the belt.

According to the above-described example structures, the rib can be madesmaller in area in contact with the recording sheet when compared withthat spreading like a plate and conveying resistance to the recordingsheet can be reduced. As a result, jamming of recording sheets can befurther prevented.

According to the above-described example structures, smooth conveyanceis realized, so that the possibility that the conveying resistancesuddenly increases and jamming occurs can be reduced.

According to the above-described example structures, the rotary bodyrotatably supported can rotate in a direction of releasing a contactforce of a recording sheet when the recording sheet comes into contact,so that the conveying resistance to the recording sheet can be reduced.As a result, jamming of recording sheets can be further prevented.

According to the above-described example structures, a recording sheetis conveyed between the guiding member and the auxiliary guiding member,so that the behavior of the recording sheet is stabilized.

According to the above-described example structures, the auxiliaryguiding member and the belt are integrally formed, so that positionalaccuracy with respect to the belt increases, and a recording sheet ismore reliably guided to the conveying surface of the belt.

In addition, by detaching the belt from the image forming apparatus mainbody, a recording sheet jamming at the auxiliary guiding member can bemore easily removed.

According to the above-described example structures, in thiselectrophotographic image forming apparatus, when air is trapped betweenthe conveying surface of the belt and a recording sheet, void may occur,however, the present invention is constructed so that air is hardlytrapped between the recording sheet and the conveying surface of thebelt as described above, so that occurrence of such a void can beprevented.

According to the above-described example structures, in such a so-calledtandem image forming apparatus, a void can be further prevented asdescribed above.

According to the above-described example structures, the photosensitivemember and the guiding member are integrally provided via a frame, sothat the positional accuracy of the guiding member with respect to thephotosensitive member is improved, and it becomes possible to accuratelyguide a recording sheet to the belt facing the photosensitive member.

In addition, by detaching the frame from the image forming apparatusmain body, a recording sheet jamming at the guiding member can be easilyremoved.

According to the above-described example structures, byelectrostatically adsorbing a recording sheet by the conveying surfaceof the belt, it becomes possible to reliably bring the recording sheetinto close contact with the conveying surface of the belt, and on theother hand, when air is trapped between the recording sheet and thebelt, around the air, the belt and the recording sheet are in closecontact with each other, so that air is hardly released. However, in thepresent invention, as described above, in the process of bringing therecording sheet into contact with the conveying surface of the belt, airbetween the sheet and the conveying surface of the belt is released, sothat trapping of air between the recording sheet and the conveyingsurface of the belt can be prevented.

According to the above-described example structures, a surface of asheet conveyed by the re-conveying mechanism opposite a surface on whichan image has already been printed comes into contact with the guidingmember, so that a possibility that the printed surface is stained ortoner scatters from the surface can be reduced.

According to the above-described example structures, the conveyingrollers which feed-out the recording sheet to the belt side are resistrollers which correct the tip end portion of the recording sheet, sothat other resist rollers are not necessary, and the constructionbecomes simple.

According to the above-described example structures, a contact angle ofthe tip end portion of the recording sheet with the guide member is setso as to be always not more than 45 degrees. When the tip end portion ofthe recording sheet is pressed against the guide member at a greatcontact angle, the load on the recording sheet increases, and the tipend portion of the recording sheet may be damaged, however, according tothe present construction, the tip end portion of the recording sheetcomes into contact with the guide member always at a gentle angle, sothat the recording sheet can be prevented from being damaged.

According to the above-described example structures, a feed-outdirection of a recording sheet from the conveying rollers is inclinedtoward a recording sheet conveying direction on the belt more than thedirection orthogonal to the recording sheet conveying surface of thebelt. Thereby, even when the apparatus is downsized, curvature of therecording sheet in the recording sheet conveying path can be reduced.

When a recording sheet being low in elasticity is used, air is trappedbetween the recording sheet and the conveying surface of the belt andeasily lowers the image quality, so that it is desirable that the widthdirection central portion of the recording sheet is curved so as toprotrude toward the conveying surface of the belt, and the recordingsheet is brought into contact with the belt from this central portion.

However, when a recording sheet being high in elasticity is used, due tothe high elasticity of the recording sheet, the recording sheet ishardly creased and trapping of air between the recording sheet and theconveying surface of the belt hardly occurs. On the other hand, if sucha recording sheet being high in elasticity is forcibly curved by a forcestronger than a predetermined force, this curve may be hardly restored.When such a recording sheet is fed to the conveying surface of the beltin this greatly curved state, even after the recording sheet is placedon the conveying surface and conveyed to a transfer position, it remainscurved, and a developing image may not be normally transferred onto therecording sheet. Accordingly, when such a recording sheet being high inelasticity is used, it is desirable that the recording sheet is notgreatly curved.

According to the above-described example structures, when a recordingsheet being low in elasticity comes into contact with the guidingmember, the guiding member is pressed to a retracted position due to theelasticity (elastic force) of the recording sheet, however, thispressing force is overcome by the urging force of the urging unit andthe guiding member is not displaced to the retracted position. As aresult, the width direction central portion of the recording sheet issufficiently curved by the guiding member so as to protrude toward theconveying surface of the belt. Thereby, it becomes possible to preventthat air is trapped between the recording sheet and the conveyingsurface of the belt and lowers the image quality.

On the other hand, when a recording sheet being high in elasticity comesinto contact with the guiding member, the guiding member is pressed tothe retracted position by the elasticity (elastic force) of therecording sheet, and the pressing force overcomes the urging force ofthe urging unit and the guiding member is displaced to the retractedposition. As a result, the recording sheet is prevented from beingexcessively curved. Thus, it becomes possible to prevent that an imageis formed by the image forming unit while leaving the recording sheethigh in elasticity curved and lowered in quality.

<Other Example Structures>

The present invention is not limited to the above-described examplestructures, and for example, the following example structures are alsoincluded in the technical scope of the invention, and moreover, otherthan the following example structures, the invention can also be carriedout by being variously changed without deviating from the aspects of theinvention. As a recording sheet on which an image may be recorded, otherthan the sheets, OHP plastic sheets or fabric sheets may be used. As thecartridge, a cartridge including a photosensitive drum and a developingdevice (developing cartridge) is shown in the example structure, and acartridge including only a developing device is shown, however, it isalso possible that the cartridge is separated from the developing deviceand provided with only a photosensitive drum.

What is claimed is:
 1. An image forming apparatus comprising: a belthaving a conveying surface on which a recording sheet is configured tobe conveyed; an image forming unit disposed opposite to the belt, theimage forming unit configured to form an image on a surface of therecording sheet conveyed by the belt; a conveying unit configured toconvey the recording sheet onto the belt; and a first guiding memberdisposed opposite to the conveying surface of the belt and configured toat least partly guide the recording sheet toward the conveying surfaceof the belt while the first guiding member comes into contact with afirst surface opposite to a second surface of the recording sheet whichcomes into contact with the conveying surface of the belt, the recordingsheet being guided by the first guiding member while the recording sheetis bent at a center of a tip end portion thereof with respect to a widthdirection orthogonal to a conveying direction of the recording sheet soas to protrude toward the conveying surface of the belt, wherein thefirst guiding member remains separated from the belt by a distancegreater than the thickness of the recording sheet; a second guidingmember configured to at least partly guide the recording sheet towardthe first guiding member while the second guiding member contacts withthe first surface opposite to the second surface of the recording sheet;a shaft extending in a width direction of the image forming apparatus;and an auxiliary guiding member disposed opposite to the first guidingmember, wherein the auxiliary guiding member includes at least one ribwhich faces the first guiding member and protrudes in a directionorthogonal to the conveying direction of the recording sheet so as to beconfigured to contact the second surface of the recording sheet, whereinthe first guiding member includes a plurality of ribs which protrude ina direction orthogonal to the conveying direction of the recording sheetso as to be configured to contact the first surface of the recordingsheet, wherein the plurality of ribs includes a first rib provided at acenter portion of the recording sheet in a width direction of therecording sheet and a second rib provided at a side portion of thecenter portion in the width direction, wherein a protrusion amount ofthe first rib is larger than that of the second rib, wherein a contactangle of the tip end portion of the recording sheet with the secondguiding member is set to be equal to or less than 45 degrees, whereinthe second guiding member includes a first terminal end at an upstreamend of the second guiding member, relative to the conveying direction ofthe recording sheet, and a second terminal end at a downstream end ofthe second guiding member, relative to the conveying direction of therecording sheet, and wherein the second guiding member is configured toengage with the shaft closer to the first terminal end of the secondguiding member than the second terminal end of the second guiding membersuch that the second guiding member is rotatable around the shaft. 2.The image forming apparatus according to claim 1, wherein the conveyingunit comprises; a conveying roller disposed on an upstream side of thefirst guiding member along the conveying direction of the recordingsheet and configured to convey the recording sheet to the belt, whereinthe second guiding member is configured to at least partly guide therecording sheet conveyed by the conveying roller toward the firstguiding member while the second guiding member contacts with the firstsurface opposite to the second surface of the recording sheet, and thesecond guiding member has such a concave shape that the second guidingmember gradually separates from and approaches the conveying surface ofthe belt in a direction orthogonal to the conveying surface of the beltfrom an upstream side to a downstream side with respect to the conveyingdirection of the recording sheet.
 3. The image forming apparatusaccording to claim 2, wherein a speed at which the recording sheet isconveyed by the conveying roller is set to be higher than a speed atwhich the recording sheet is conveyed by the belt.
 4. The image formingapparatus according to claim 2, wherein the conveying roller is a resistroller configured to correct the tip end portion of the recording sheet.5. The image forming apparatus according to claim 2, wherein a feed-outdirection of the recording sheet from the conveying roller is inclinedtoward the conveying direction of the recording sheet on the belt morethan a direction orthogonal to the conveying surface of the belt.
 6. Theimage forming apparatus according to claim 1, wherein a protrusionamount of the first rib is configured to gradually increase from theupstream side toward the downstream side along the conveying directionof the recording sheet.
 7. The image forming apparatus according toclaim 1, wherein the first guiding member comprises a rotary bodyrotatably supported at a position configured to be opposite to a centerof the recording sheet with respect to the width direction of therecording sheet orthogonal to the conveying direction of the recordingsheet, the rotary body protruding so as to come into contact with thefirst surface opposite to the second surface of the recording sheetwhich is opposed to the conveying surface of the belt.
 8. The imageforming apparatus according to claims 1, wherein the auxiliary guidingmember is configured to guide the tip end portion of the recording sheettoward the conveying surface of the belt by coming into contact with thesecond surface opposite to the first surface of the recording sheetwhich comes into contact with the first guiding member.
 9. The imageforming apparatus according to claim 8, wherein the belt is detachablefrom a main body of the image forming apparatus and the auxiliaryguiding member is integrally provided with the belt.
 10. The imageforming apparatus according to claim 1, wherein the image forming unitcomprises: at least one photosensitive member on which a developingimage is configured to be formed being disposed opposite to theconveying surface of the belt; an exposing unit configured to expose asurface of the photosensitive member to form an electrostatic latentimage on the surface of the photosensitive member; a developing unitconfigured to form a developing image that forms an electrostatic latentimage by supplying a developer onto the surface of the photosensitivemember on which the electrostatic latent image is formed; and a transferunit configured to transfer the developing image formed on the surfaceof the photosensitive member onto the surface of the recording sheetconveyed on the conveying surface of the belt when applied with atransfer bias.
 11. The image forming apparatus according to claim 10,wherein the at least one photosensitive member comprises a plurality ofphotosensitive members that are disposed along the conveying directionof the recording sheet.
 12. The image forming apparatus according toclaim 10, wherein, in the image forming unit, the at least onephotosensitive member is supported by a frame detachably provided in amain body of the image forming apparatus, and wherein the first guidingmember is provided integrally with the frame.
 13. The image formingapparatus according to claim 1, comprising: an electrostatic adsorbingunit configured to electrostatically adsorb the recording sheet guidedby the first guiding member by the conveying surface of the belt whenapplied with a bias.
 14. The image forming apparatus according to claim1, comprising: a re-conveying mechanism configured to reverse therecording sheet on which an image is formed by the image forming unitand to feed back the recording sheet to the conveying unit.
 15. Theimage forming apparatus according to claim 1, wherein the first guidingmember is displaceably supported between a first position where thefirst guiding member is projected toward a surface of the conveyedrecording sheet and a second position where the first guiding member isretracted farther than the first position from the surface of therecording sheet, and wherein the first guiding member comprises anurging unit that urges the first guiding member toward the firstposition.
 16. The image forming apparatus according to claim 1, whereinthe first guiding member is displaceably supported between a firstposition where the first guiding member is projected toward the surfaceof the conveyed recording sheet and a second position where the firstguiding member is retracted farther than the first position from thesurface of the recording sheet, wherein the first guiding member is atthe first position when contacted by a sheet being as low in elasticityas the recording sheet, and wherein the first guiding member is at thesecond position when contacted by a sheet being higher in elasticitythan that the sheet being as low in elasticity as the recording sheet.17. The image forming apparatus according to claim 1, wherein a portionof the first guiding member configured to guide the recording sheet isdisposed opposite to the second guiding member and faces away from theconveying surface of the belt.
 18. The image forming apparatus accordingto claim 1, further comprising: a main body casing formed with a manualfeed side paper feed path on the upstream end of the second guidingmember along the conveying direction of the recording sheet, wherein theconveying unit comprises; a conveying roller disposed on the upstreamend of the second guiding member along the conveying direction of therecording sheet and configured to convey the recording sheet to thebelt; and a paper dust removing roller disposed on an upstream end ofthe first guiding member along the conveying direction of the recordingsheet, and wherein a feed-out direction of the recording sheet from theconveying roller is a direction between a sheet inserting direction intothe manual feed side paper feed path and the feed-out direction of therecording sheet from the paper dust removing roller.
 19. The imageforming apparatus according to claim 1, wherein the first guiding memberis integrally provided with the second guiding member.